NASA's Chandra X-ray Observatory has spotted a curious
outburst from our galaxy's core -- a sign that the Milky Way's
central black hole may be snacking on its neighbors.

Sept.
5, 2001: Astronomers have long suspected that the Milky Way
harbors a "monster in the middle" -- that is, a supermassive
black hole at the very center of our pinwheel galaxy. Other galaxies
have one, they reasoned, so ours probably does, too.

But finding it hasn't been easy. Light (by definition) can't
escape a black hole, so the Milky Way's central monster has remained
elusive.

Right: An artist's concept of matter swirling,
like water down the drain of a bathtub,
into a supermassive black hole. Courtesy of the Chandra X-ray
Observatory's X-ray
Astronomy Field Guide.

Undaunted, observers have been peering into the heart of the
Milky Way for many years. The fast motions of stars and gas around
the galactic center suggested that something very massive indeed
was hidden there. But what? If it was a black hole, X-ray
telescopes ought to detect a telltale glow from super-heated
gas swirling into the hole -- what astronomers call an "accretion
disk." Astronomers had spotted such X-ray emitting disks
at the hearts of other galaxies, but not inside the Milky Way.
It was a real puzzle.

So, when NASA launched the Chandra X-ray Observatory
a little more than two years ago, a team of astronomers led by
MIT's Fred Baganoff wasted no time turning the sensitive telescope
toward the galactic center. And in September, 1999, they found
what they were looking for: faint X-ray emissions from fiery
gas circling a black hole. The
monster, massing nearly three million times more than our
own Sun, was real.

Now, they may have caught the black hole in the act of devouring
something, too.

On October 26, 2000, Baganoff and colleagues were once again
using Chandra to monitor the center of our galaxy when they recorded
a powerful X-ray outburst. They were looking toward "Sagittarius
A*," an intense radio source that astronomers believe is
powered by the black hole. During a span of just a few minutes,
X-ray emissions from Sagittarius A* became 45 times brighter
than normal, before declining to pre-flare levels a few hours
later.

"This is extremely exciting because it's the first time
we have seen our own neighborhood supermassive black hole devour
a chunk of material," said Baganoff. "It's as if the
material there sent us a postcard just before it fell in."

The energy released in the flare corresponds to a sudden infall
of material with about as much mass as a comet or an asteroid.
The black hole literally gobbled something up! On the other hand,
say scientists, the flare might have been caused by the reconnection
of magnetic field lines near the black hole -- a process that
also triggers solar flares on the Sun.

Above: This false-color image shows the central region
of our Milky Way Galaxy as seen by Chandra. The bright, point-like
source at the center of the image was produced by a huge X-ray
flare that occurred in the vicinity of the supermassive black
hole at the center of our galaxy. Credit: NASA/MIT/F. Baganoff
et al. [more]

In either case, shock waves from the explosion would have
accelerated electrons near the black hole to nearly the speed
of light -- leading to the observed outburst of X-rays. A longer-term
increase in radio emission was also recorded beginning around
the time of the flare, indicating that, indeed, the production
of high-energy electrons was increasing.

At the peak of the flare, the X-ray intensity dramatically
dropped by a factor of five, then recovered, all within a 10-minute
interval. Such fluctuations constrain the size of the emitting
region to be no larger than about 20 times the size of the event
horizon -- the one-way membrane around a black hole predicted
by Einstein's theory of relativity.

"The
rapid variations in X-ray intensity indicate that we are observing
material that is as close to the black hole as the Earth is to
the Sun," said Gordon Garmire of Penn State University,
principal investigator of Chandra's Advanced CCD Imaging Spectrometer,
which was used in these observations.

"This signal comes from closer to the event horizon of
our galaxy's supermassive black hole than any that we have ever
received before," agreed Baganoff. Indeed, X-ray observations
of Sagittarius A* seem to offer a unique way to probe activity
very close to such black holes where space and time itself are
twisted.

It's a prospect that thrills astronomers who have been waiting
a long time for such data. Consider that the first
discussions of black holes -- by the French physicist Pierre
Laplace and the English philosopher John Michell -- date back
to the 18th century. But until recently we couldn't even find
the biggest one in our own galaxy! Thanks to Chandra's sensitive
X-ray cameras, however, black holes are finally revealing their
secrets, a well-earned reward for patient astronomers.

Editor's Note: This week X-ray astronomers are celebrating
Chandra's 2nd successful year of discoveries as well as the good
news from NASA headquarters that the Observatory's mission has
been doubled from 5 to 10 years. For more information visit http://chandra.nasa.gov or
http://chandra.harvard.edu.